Such an engine type is better described in U.S. Pat. No. 5,975,050 wherein the engine operates under single-fuel combustion conditions with a fuel generally in the liquid phase, of diesel type, injected directly into the combustion chamber of the engine (direct injection) or into a precombustion chamber (indirect injection), or under dual-fuel combustion conditions, wherein a gaseous fuel is associated with this liquid fuel. This mode allows to achieve two combustion modes in the same engine cycle, fuel self-ignition and combustion of the air/gas mixture through a flame front propagation initiated by self-ignition.
As it is known from the prior art, methods and devices allow to control the fuel distribution and the transition between the two operating modes of the engine. By way of example, U.S. Pat. Nos. 5,450,829 and 5,975,050 and international applications WO-99/45,256 and WO-99/46,495 describe such methods and devices.
A method of controlling an internal-combustion engine operating in single-fuel or dual-fuel mode is also known from French patent FR-2,817,913 filed by the applicant. With this method, during operation in single-fuel mode, the energy required for operation of the engine is provided either by indirect injection of a gaseous fuel or by direct injection of a liquid fuel, gasoline here. During operation in dual-fuel mode, a gasoline injection is carried out for a mixture in a specific form while being combined with a gaseous fuel injection.
However, the applicant has observed that the various fuels used, such as gasoline or natural gas mainly, have quite different physico-chemical characteristics. Thus, two of these characteristics, which are the octane number (antiknock power) and the volume energy density (energy that can be developed for a volume of air-fuel mixture at a given fuel/air ratio, pressure and temperature), have a great impact on the engine performances and efficiency.
These differences between the fuels used in a single engine lead to define the characteristics of the engine—such as its compression ratio, its valve timing (opening and closing of the intake and exhaust valves), or the definition of the supercharger in case of a supercharged engine—according to either one of the fuels that can be used or for an operating compromise with the various fuels used.
In all the aforementioned cases, the engine does not run under optimum conditions.
In fact, in the case of a supercharged spark-ignition engine that can operate with a single-fuel or a dual-fuel combustion mode, with a fuel such as natural gas and/or gasoline, the natural gas is injected in gaseous form into the intake pipes by means of its feed circuit (tanks, feed ramp and specific injection nozzles) and the gasoline is injected in liquid form either into the engine intake pipes (indirect injection) or directly into the combustion chamber (direct injection) by means of a second feed circuit suited to the characteristics thereof.
This engine type is perfectly known to the person skilled in the art, but the engine performances are limited by the characteristics of each fuel, i.e.: the energy density for natural gas or the octane number for gasoline.
In fact, in the case of natural gas, the octane number of the gasoline is very high (of the order of 120 to 130). This allows to improve the engine efficiency by disposing of the engine knock problem, i.e. self-ignition of the fuel mixture (or charge) that can lead to engine destruction, through an increase in the compression ratio or optimum adjustment of the combustion timing or fuel/air ratio limitation in order to control the exhaust temperature. On the other hand, this natural gas, which is injected in gaseous form and has a low energy density, of the order of 3.1 kJ/l (kilojoules per liter), leads to limit the engine performances through a volumetric efficiency loss that cannot be always compensated by supercharging.
The engine therefore has a good efficiency but limited performances, notably at low engine speeds.
Conversely, when the engine runs on gasoline, the octane number of the fuel is much lower (of the order of 91 to 98) but the energy density is high (above 3.4 kJ/l). This provides good energy filling of the combustion chamber but with a low use efficiency.
The engine therefore has a reduced performance linked with the limitation imposed by the engine knock, either in terms of usable volumetric compression ratio, or in terms of combustion timing through sub-ignition advance.
The engine performance is consequently limited by the most critical parameter of the fuel used: the energy density for natural gas or the octane number for gasoline.
The present invention aims to overcome the aforementioned drawbacks by means of a control method allowing to maximize exploitation of the strong points of each fuel.